A hydrostatic transmission using two hydraulic engines one of which acts as a hydraulic pump when the other serves as a hydraulic motor finds wide application, because it can change speed continuously. Further, many transmissions of this kind have been studied and developed. A rotary fluid energy converter disclosed in Japanese Patent Laid-Open No. 77179/1983 is an example of such a hydraulic engine. Another conventional hydraulic transmission makes use of a combination of a piston pump and a motor of either the swash plate type or the bent axis type. In many conventional systems, only the pump has a variable displacement and the motor has a fixed displacement. Such an arrangement only allows a narrow range of speeds to be obtained. Further, even in systems where the pump and the motor have variable displacements, the two displacements are frequently varied independently of one another. This renders the control over the system complex.